A camera representation is missing from my scene. How can I see it?
This is a very common issue for newer Blender users. There are many hotkeys that they are not aware of, and because of that may unwittingly do something not intended. 
This is a prime example of that. You appear to have accidentally pressed H while your camera was selected. This keystroke is the Blender shortcut to hide objects in the viewport. To make all objects in the scene visible again, press Alt+H. This un-hides all objects in the current viewport.